The basic construction and operation of the electromagnetic clutches which are usable for controlling the power transmission between the automobile engine and the refrigerant compressor to selectively drive the compressor are well known. Examples of such basic construction and operation are found in prior patents, such as U.S. Pat. Nos. 3,044,594 and 3,082,933.
Referring to FIG. 1 herein, a prior art electromagnetic clutch will be described. FIG. 1 is a cross-sectional view of a known electromagnetic clutch which is mounted on a refrigerant compressor. The electromagnetic clutch is disposed on the outer peripheral portion of a tubular extension 2 projecting from an end surface of a compressor housing 1. Tubular extension 2 surrounds a drive shaft 3 of the compressor. Drive shaft 3 is rotatably supported in compressor housing 1 through bearings (not shown). The electromagnetic clutch includes a clutch rotor which is rotatably mounted on tubular extension 2 through a bearing 4, and is connected to an automobile engine (not shown) through a belt 6. Rotor 5 is thus subjected to strong tension forces. Rotor 5 is provided with a plurality of concentric arcuate magnetic pole faces 5a at its axial end. Outer terminal end of drive shaft 3 extends from tubular extension 2, and a hub 7 is fixed on the extending terminal end of drive shaft 3. An annular armature plate 8 is flexibly joined by a plurality of leaf springs 9 to hub 7 in such a fashion that armature plate 8 faces pole faces 5a of rotor 5 with a predetermined axial air gap between plate 8 and pole faces 5a. An electromagnet 10 is mounted on compressor housing 1 and is concentric with drive shaft 3. Electromagnet 10 is fixed within an annular hollow portion 5b formed in rotor 5 with a surrounding air gap to supply the magnetic flux for attracting armature plate 8 to the pole faces 5a of rotor 5.
Thus, when an electromagnetic coil 101 of electromagnet 10 is energized, armature plate 8 is attracted to concentric pole faces 5a of rotor 5. Drive shaft 3 is then rotated together with rotor 5 by the engine output through leaf springs 9 and hub 7. When electromagnetic coil 101 of electromagnet 10 is not energized, armature plate 8 is separated from pole faces 5a of rotor 5 due to elasticity of leaf springs 9. Rotor 5 is thus rotated by the engine output, but the compressor is not driven.
In these prior electromagnetic clutches, rotor 5 consists of an outer cylindrical portion 51 formed with at least one V-shaped groove 51a for receiving belt 6, an inner cylindrical portion 52, and an axial end annular plate portion 53 which connects outer and inner cylindrical portions 51. The concentric pole face 5a are formed in the axial end surface of annular plate portion 53. This rotor 5, as shown in FIG. 1, has been formed as an integral body of magnetic material by forging followed by machining. But the resultant rotor 5 is heavy, so that the total weight of the compressor having the electromagnetic clutch is relatively large. This means that the load on the drive source, such as the automobile engine, is increased. Furthermore, since the forging process only forms a preformed or rough rotor, an amount of rotor material must be machined to form the final, accurately dimensioned rotor, with the result that relatively large quantities of waste metal are produced. Such forging and machining of the rotor also consumes a great deal of time.
In order to avoid these disadvantages and to obtain the light weight clutch rotor, another prior art rotor, rotor 5, shown in FIG. 2, has been made. In rotor 5' an annular magnetic body 51' and a annular V-shaped groove member 52' are separately produced by a press forming method. In this construction, main annular body 51' and V-shaped groove member 52' are brazed to one another after main annular body 51' is fitted into V-shaped groove member 52'. In this method, welding material is unequally deposited between main annular body 51' and groove member 52', resulting in an unbalance of rotor 5'. Furthermore, main annular body 51' of rotor 5' is formed by cold forging, which requires the use of a large press machine. In this method, the main annular body 51' must be passed through a plurality of working processes, which include an annealing process to remove the strain on the main annular body 51' caused by such cold forging to form the final, accurately dimensioned rotor 5'. The forming process of the rotor is thus complicated.
In order to avoid the above disadvantages, another prior art rotor, rotor 5" shown in FIG. 3, has been made. In rotor 5" an L-shaped magnetic body 51" and an annular V-shaped groove member 52" are separately formed by a press forming method. In this construction, L-shaped magnetic body 51" consists of inner cylindrical member 51a" and an axial end annular plate portion 51b" and both member 51b" and 52" are brazed to one another. Since the mating surfaces between magnetic body 51" and V-shaped groove member 52" are brazed, the strength of the rotor at the mating surfaces is unstable. Rotor 5" is also unbalanced because of the difficulty in obtaining a uniform brazed connection between the mating surfaces.